The present invention relates generally to optical devices. The present invention relates more particularly to a high performance filter or interleaver for optical communications and the like.
Optical communication systems which utilize wavelength-division multiplexing (WDM) and dense wavelength-division multiplexing (DWDM) technologies are well known. According to both wavelength-division multiplexing and dense wavelength-division multiplexing, a plurality of different wavelengths of light, preferably infrared light, are transmitted via a single medium such as an optical fiber. Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels. The plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system. Thus, according to wavelength-division multiplexing and dense wavelength-division multiplexing technologies, a much greater amount of information can be transmitted than is possible utilizing a single wavelength optical communication system.
The individual channels of wavelength-division multiplexed or dense wavelength-division multiplexed signals must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof. This separation or demultiplexing process can be performed or assisted by a filter or an interleaver. A similar device facilitates multiplexing of the individual channels by a transmitter.
It is important that the interleavers separate the individual channels sufficiently so as to mitigate undesirable crosstalk therebetween. Crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel. As those skilled in the art will appreciate, such crosstalk interferes with the detection and/or demodulation process. Generally, the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signals.
Although configuring an interleaver so as to have a wide passband is generally desirable, so as to facilitate the filtering of signals which have drifted somewhat from their nominal center wavelength, the use of such wider passbands introduces the possibility for undesirably large dispersion being introduced into a filtered channel. Typically, the dispersion introduced by a birefringent filter or interleaver increases rapidly as the channel spacing is reduced and as a channel moves away from its nominal center wavelength. Thus, as more channel wavelength error is tolerated in a birefringent filter or interleaver, greater dispersion values are likely to be introduced.
As those skilled in the art will appreciate, excessive dispersion limits the ability to reliably demodulate such optical signals. Therefore, dispersion tends to place a limit on how closely channels can be spaced in an optical communication system and thus tends to limit the efficiency of the utilization of available bandwidth.
As channel usage inherently increases over time, the need for efficient utilization of available bandwidth becomes more important. Therefore, it is highly undesirable to increase channel spacing and/or to reduce communication speed in order to compensate for the effects of crosstalk and dispersion. Moreover, it is generally desirable to decrease channel spacing and to increase communication speed so as to facilitate the communication of a greater quantity of information utilizing a given bandwidth.
Modern dense wavelength-division multiplexed (DWDM) optical communications and the like require that network systems offer an ever-increasing number of channel counts, thus mandating the use of a narrower channel spacing in order to accommodate the increasing number of channel counts. The optical interleaver, which multiplexes and demultiplexes optical channels with respect to the physical media, i.e., optical fiber, offers a potential upgrade path, so as to facilitate scalability in both channel spacing and number of channel counts in a manner which enhances the performance of optical communication networks.
As a multiplexer, an interleaver can combine two streams of optical signals, wherein one stream contains odd channels and the other stream contains even channels, into a single, more densely spaced optical signal stream. As a demultiplexer, an interleaver can separate a dense signal stream into two, wider spaced streams, wherein one stream contains the odd channels and the other stream contains the even channels. Thus, the interleaver offers scalability which allows contemporary communication technologies that perform well at wider channel spacing to address narrower, more bandwidth efficient, channel spacings.
There are four basic types of interleavers suitable for multiplexing and demultiplexing optical signals. These include birefringent filters, thin-film dielectric devices, planar waveguides, and fiber-based devices. All of these contemporary interleaving technologies suffer from substantial limitations with respect to channel spacing, dispersion, insertion loss, channel isolation, temperature stability, cost, reliability and flexibility. Thus, there is a need to provide an optical interleaver which can overcome or mitigate at least some of the above-mentioned limitations.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a dispersion mitigating interleaver assembly comprising a first unbalanced Mach-Zehnder interferometer assembly which includes first and second output ports and which has a first transmission vs. wavelength curve and a first dispersion vs. wavelength curve. The dispersion mitigating interleaver assembly also includes a second unbalanced Mach-Zehnder interferometer assembly which has a second transmission vs. wavelength curve and a second dispersion vs. wavelength curve. The second unbalanced Mach-Zehnder interferometer assembly receives an output from one of the first and second output ports of the unbalanced Mach-Zehnder interferometer assembly. The second transmission vs. wavelength curve is substantially the same as the first transmission vs. wavelength curve and the second dispersion vs. wavelength curve is substantially opposite with respect to the first dispersion vs. wavelength curve, such that dispersion is substantially cancelled by the cooperation of the first and second unbalanced Mach-Zehnder interferometer assemblies.
These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.